osmocon: Use libosmocore logging system

change the built-in printf system with libosmocore's logging system. No VTY cfg is required, only ability to set debug level and enable/disable timestamp through cmd line params like arleady done in other osmocom projects.

Specially interesting for this task is to have timestamps printed in the log, so we can cross-check information with other logs in TTCN3 BTS_Tests using real hardware.